Tuesday, June 5, 2012

Venus Transits Sun Today - Be Careful How You Observe It!

Today the planet Venus passes across the face of the Sun or what we call a "transit". Transits are rare because to be deemed so from Earth perspective they require Venus' path be very nearly in Earth's orbital plane in relation to the Sun.  This nearly happens each time at what we call "inferior conjunction, see e.g. the diagram to the right in this earlier blog:


The problem is that Venus' own orbit tilts 3.4 degrees to the plane of Earth's orbit so in reality it usually passes either slightly above or below the Sun. Meanwhile, the orbital planes of Earth and Venus intersect along the so -called "line of nodes". Earth crosses this line twice a year - near June 7, and December 8. Transit occurs only when Venus reaches inferior conjunction within a day or two of these dates.

Despite transits being relatively rare more than half the world will get to see this year's event. For most of North America the transit begins in the afternoon and continues through sunset. For the great proportion of interested people the best opportunity to view will be at a local Observatory, or by way of telescopes and special equipment set up for the purpose by a local amateur astronomical society. Whatever you do, it is NOT a good idea to try to view it directly, without protection!

Bear in mind your eye lens is converging and hence if you were to gaze directly at the Sun the lens action will be to act as a magnifer lens and burn a hole into your retina.  Even without any other optical aid, direct sunlight can burn your retina in seconds. This is a price too high to pay, even for an event which won't recur until December 2117.

A better option, if you decide to go it alone, is to use No. 14 Welder's glass or specially made "eclipse glasses" designed for viewing the Sun at time of solar eclipse. Both systems will block dangerous UV and infrared radiation as well as visible light. Welder's glass can be purchased at most welding supply stories.

Now, given that Venus disk occupies 57."8 (arcsecs) during this transit this means it will cover barely 3 % of the Sun's 1890" width disc.With eyes alone the planet will therefore appear as an obvious but small spot as it moves across the northern half of the solar disc.

An alternative way to view on your own is by projecting the Sun's image . Thus, if you use binoculars, simply line up the projected solar image so it appears on a piece of white cardboard at the eyepiece end. (Generally you will need to hold it a foot or two from the end of the eyepiece). The projected solar image appears larger (but dimmer) the farther away you hold the cardboard.

The best way and most close up views will be had by way of observing directly through a telescope (refractor or reflector) outfitted with special solar filters.

This is why I suggested first trying to locate an amateur society in your area that will be viewing the event and going there to see it.

Good seeing!

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